1. Field of the Invention
The present invention relates to an image forming apparatus in which a toner image formed on an image carrier is transferred onto an intermediate transfer member (primary transfer) once and then transferred onto a transfer material (second transfer) by using an electro-photographic method.
2. Description of Related Art
An image forming apparatus having a configuration described below is well known.
Namely, a developing section including a photosensitive material drum (an image carrier), a charging section, an exposing section and a developing section for magnetically attracting developer composed of carrier and toner onto a rotating sleeve and transferring to a developing area is provided for each color of yellow, magenta, cyan and black.
Then, when forming an image, after forming a toner (powder) image according to the image data for each color onto the photosensitive material drum, these toner images are sequentially transferred (primary transfer) onto a belt shaped and rotating intermediate transfer member, which is synchronized with the photosensitive material drum to form a superimposed color toner image onto the intermediate transfer member.
Next, the second transfer section structured by a backup roller capable of rotating, provided inside the intermediate transfer member and a second transfer roller provided outside of the intermediate transfer member transfers (second transfer) the color toner image onto the transfer material such as paper sheets.
In such an image forming apparatus, for example, the toner image formed on the photosensitive material drum can be obtained by reversal development. On this development stage, there is a case that toner happens to adhere on the outside area on which the toner image is formed (an area of the front and rear side of the image area viewing the photosensitive material drum in the rotational direction).
This excessive toner moves to the intermediate transfer member, and further moves and adheres on the second transfer section such as the surface of the second transfer roller. This excessive toner dirties the rear surface of the transfer material, which is conveyed to the second transfer area, following to the preceding transfer material. As a result, this problem causes to damage of the quality as products.
Particularly, in the color image forming apparatus as described above, comparing with, for example, a monochrome (single color) image forming apparatus, the quantity of toner for adhering the outside of the image area of the intermediate transfer member becomes large. Thus, how to remove the toner from the second transfer roller is extremely big issue.
Japanese Patent Application Open to Public Inspection No. H03-248181 disclosed a following method in order to suppress the problems described above.
In aforementioned Japanese Patent Application, an image forming apparatus having no intermediate transfer member was disclosed as an example. However, toner adheres on the surface of the transfer rollers corresponding to the second transfer roller described above and the problems caused by the toner is the same as described above. The method for solving the problems can be basically applied to the image forming apparatus having an intermediate transfer member.
According to the method disclosed in aforementioned Japanese Patent Application, the method comprises the steps of
passing a transfer material into a transfer section being a nip section structured by an image carrier and a transfer section touching to the image carrier,
applying bias voltage having a polarity, which is reverse to the bias voltage at a time when the transfer material exists in the transfer section, when transfer material does not exist in the transfer section at no transfer operation period, and
keeping a polarity of residual toner on a surface of the transfer section entering into the transfer section reverse to the polarity of the transfer bias voltage at least while the transfer material is passing through the transfer section.
Based on this arrangement, in the image forming apparatus utilizing a contact type transfer section such as transfer rollers, suppressed are the contamination in the apparatus and dirt on the rear side of the transfer material caused by the toner adhering to the transfer section.
With regard to the toner adhesion and removal to and from the transfer section, further concrete behavior of the toner will be described below.
Namely, when taking the method for visualizing a latent image on the image carrier charged in minus by using the toner charged in plus, the transfer process is conducted by applying a minus transfer bias voltage to the transfer roller when conducting the transfer operation and a plus bias voltage is applied to the transfer roller after completing the transfer operation.
Even though a part of toner on the transfer roller can be returned to the image carrier when non-transferring, due to the discharge at the transfer section, other toner is charged in minus and resides on the transfer roller.
With regard to the residual toner, toner charged in minus returns to the image carrier by applying minus bias voltage to the transfer roller again. However, a part of toner is charged in plus and as a result the other toner resides on the transfer roller.
By maintaining this state until the transfer roller makes one rotation, almost all of the toner charged in minus can be returned to the image carrier side.
As described above, the polarity of the charged toner on the transfer roller becomes reverse to the polarity of transfer bias voltage.
The transfer operation to the next transfer material is conducted after becoming to this situation. However, since the toner charged in minus is in a situation where the toner charged in minus has been electro-statically absorbed on the transfer roller, it does not dirty the rear surface of the transfer material.
However, according to the cleaning method of the transfer roller disclosed in aforementioned Japanese Patent Application, namely, the method for changing the bias voltage applied to the transfer roller when conducting the transfer operation and the non-transfer operation, the dirt on the rear surface of the transfer material can be prevented. However, following problems described below occur.
When conducting the transfer operation, the transfer bias voltage (minus voltage) is applied to the transfer roller. However, since the bias voltage, which is reverse to the transfer bias voltage, is applied to the transfer roller when conducting non-transfer operation, the electric charge caused by the bias voltage at the time of non-transfer operation has a role for canceling the bias voltage at the time of transfer operation. As a result, the efficiency of the voltage or current of the transfer bias becomes very bad.
As described above, in Japanese Patent Application Open to Public Inspection No. H3-248181, since switching operation of the bias voltages against the transfer roller is conducted, bias voltages before and after the switching operation affect each other and electric field of transfer operation becomes unstable.
Further, in the Japanese Patent Application Open to Public Inspection No. H3-248181, electric charging by a corotron charging section is conducted against the transfer roller, and switching to the transfer bias voltage is conducted while the electric charging is conducted. However since the polarity of the transfer bias voltage is an opposite polarity of the electric charging by the corotron charging section, the transfer bias voltage becomes further unstable.
An object of the present invention is to solve the problems associated with the prior art in which the transfer roller is cleaned as described above, and to prevent the dirt on the rear surface occurred in the transfer section appropriately.